A pipe "flange" is a metal ring, typically welded to the end of a pipe, with holes bored parallel to the pipe's center line to accommodate many bolts:
Before tightening the bolts, pressure-tight flange joints are created by placing a donut-shaped gasket between the flange pairs. Gaskets are made from softer materials than flange materials. The gasket will be "crushed" between two flanges in order to seal all potential leak routes.
Here is a photograph of a Rosemount magnetic flowmeter mounted with 4-bolt flange fittings:
Close inspection of the flanged connections reveals the space between the flange faces caused by the gasket material "sandwiched" between the flange pairs.
In the following image, two huge pipe flange couplings are visible on either end of a rather short "spool" pipe piece. The enormous number of studs holding each flange set together provides an indicator of the pressure of the fluid inside, which in this case exceeds 1,000 PSI!
Similar to the flowmeter flanges seen above, the gaps between the flange ring faces display the space occupied by the gasket that forms a pressure-tight seal between the flange surfaces.
A popular method for installing such a flange gasket is to install only half of the bolts (in the holes below the pipe's centerline), drop the gasket between the flanges, insert the remaining bolts, and then tighten all bolts to the appropriate torques:
Flanges vary in their sealing design and needed gasket material. One of the most prevalent flange "face" designs in the United States is the raised-face (RF) flange, which is intended to seal against a gasket via a series of concentric grooves cut on the flange's face. These grooves create a sealing surface with a significantly longer leakage path than if the faces were smooth, so discouraging the leakage of pressurized process fluid.
Another sort of flange face is known as a ring-type joint (RTJ). In this design, an unique metal ring fits into a groove carved into the faces of both mating flanges, compressing and filling the groove when the flanges are properly tightened. RTJ flanges are often used in high-pressure situations where controlling leakage is more difficult. To achieve adequate sealing, RTJ flange grooves must be totally free of extraneous material and well-formed (not deformed).
ANSI (American National Standards Institute) standard 16.5 defines a system of "pressure classes" for rating flanges in the United States. These pressure classes are identified by numbers followed by "pound," "lb," or "#." Common ANSI pressure classes include 150#, 300#, 400#, 600#, 900#, 1500#, and 2500#. Notably, these class numbers do not immediately correspond to pressure ratings in PSI, but they do scale with pressure (e.g., a 600# flange will have a higher pressure rating than a 300# flange, all other factors being equal). Not only do pressure ratings depend on the "class" of the flange, but also on the working temperature, as metals tend to become more brittle at higher temperatures. Originally, the ANSI class designations were based on the steam line service ratings of these flanges. A 250# flange, for example, was rated as such because it was intended for use in piping service with saturated steam at 250 PSI (and 400 degrees Fahrenheit).
With the advancement of metallurgy, these flanges became capable of withstanding larger pressures at higher temperatures, but the initial "pound" rating remained unchanged. This situation is comparable to the "tonnage" rating of American light trucks: a "one-ton" truck is capable of towing significantly more than 2,000 pounds of freight. The "one-ton" designation refers to a specific design that was once certified for around 2,000 pounds, but due to advancements in metallurgy and manufacture is now capable of carrying significantly more than that capacity.
To perform properly, piping flanges and components must have matching flange ratings and diameters. For instance, a control valve with a flanged body rated as a 4-inch ANSI class 300# pipe flange can only be connected to another 4-inch ANSI class 300# pipe flange. If mismatched pressure-class flanges are coupled together, the physical integrity of the pipe system will be compromised. Additionally, appropriate gasket types must be selected to correspond with the pressure class of the mating flanges. Thus, each flanged junction must be viewed as a whole system, with its integrity only guaranteed if all of its components are designed to work together.
When tightening the bolts that connect two flanges together, it is crucial to equally distribute the bolt pressure such that no one section of the flange receives significantly more pressure than any other location. In a perfect world, you would simultaneously tighten all bolts to the same torque limit. Due to the impossibility of achieving this with a single wrench, the best choice is to tighten the nuts sequentially in stages of increasing torque. The following diagram illustrates a torque sequence (the numbers indicate the order in which the bolts should be tightened):
Using a single wrench, you would apply a preliminary torque to each bolt in the order depicted. Then, the tightening operation would be repeated with increased torque for additional cycles until all bolts had been tightened to the specified torque value. Observe how the torque sequence alternates across the four quadrants of the flange, ensuring that the flanges are crushed equally as each bolt is gradually tightened. Cross-torquing is the common term for this technique of alternating quadrants around the circle.
There are specialized wrenches called torque wrenches for measuring torque applied during the tightening ope
When dealing with flanged pipe connections, it is essential to remove the bolts on the far sideration. In high-pressure, essential applications, the actual stretch of each flange bolt is monitored as a direct indicator of the bolting force. A particular bolt marketed under the brand name Rotabolt incorporates its own strain indicator, allowing the mechanic to determine whether the bolt has been adequately tightened regardless of the tool used. of the flange before loosening the bolts r the pipe to first loosen flange bolts on the opposite side, if there is any pressure inside the pipe, it should leak there first, venting away from you.on the side of the flange closest to you. This is merely a precaution against process fluid splashing over your face or body in the event of pressure buildup within a flanged pipe. By reaching ove
A unique feature of flanged pipe connections is the ability to insert a blank metal plate referred known as a blind over or between the flange faces, thus blocking flow. This is beneficial when a pipe must be blocked semi-permanently, such as when a pipe portion has been decommissioned or when a pipe section must be shut for safety reasons during maintenance operations.
To install a blind, the flange joint must be damaged, then the flanges must be pryed apart to create the appropriate space. After replacement gaskets and the blind have been placed, the flanged bolts can be reinstalled and torqued to the specified value. Here is a shot of a stainless-steel blind (not mounted on a pipe), with two welded lifting tabs plainly visible to assist handling of this heavy hardware:
In applications where "blinding" occurs frequently, a permanent type of blind known as a spectacle blind may be installed to facilitate the task. A spectacle blind consists of a standard blind plate linked by a short tab to an equal-diameter ring whose outline resembles a pair of spectacles:
The piping system can be designed and constructed with the blind's thickness in mind, with the flange-to-flange gap remaining constant in both the "open" and "blinded" states. This is particularly useful in extremely large piping systems, because the force necessary to separate previously mated flange faces may be quite great.
The following image depicts a spectacle blind placed such that the yellow-painted "blind" half is exposed and the "open" half is sandwiched between the pipe flanges to allow flow through that pipe:
This following image depicts a spectacle blind put in the opposite direction, with the "open" half exposed and the "blind" half restricting fluid flow via the pipe:
